It has been shown that flexible channelization, whereby wireless stations adapt their spectrum bands on a per-frame basis, is feasible in practice. It is known that flexible channelization has the potential to drastically increase the efficiency, fairness and load-balancing properties of wireless networks. In particular, it provides the following advantages. First, adding frequency domain decisions to the contention resolution process can mitigate severe time-domain overheads of many communication standards, such as 802.11, which are exacerbated by recent physical (PHY) layers. Second, adapting the amount of consumed spectrum becomes crucial to avoid interference in communication systems, for example in recent 802.11 amendments such as 802.11ac, which can use large channel bandwidths (up to 160 MHz) and currently requires very careful spectrum planning. Third, modulating spectrum on a per-frame basis departs from the usual static channel assignment perspective, and enables spectrum-allocation schemes to finely adapt to instantaneous traffic loads.
Despite important promises in terms of performance improvements, finding efficient schedules in time and frequency domains is difficult. It requires that different stations of a communication system reach some level of coordination, because for each frame they need to choose “time-spectrum blocks” which (i) do not overlap (to avoid interference) and (ii) consume as much of the available spectrum as possible (to maximize performance). For this reason, currently known schemes for flexible channelization rely on different forms of explicit signaling, synchronization, spectrum scanning or central control, in order to coordinate neighboring stations and efficiently organize transmissions. Employing such extra signaling introduces extra overhead and complexity, and typically adapts poorly to variable traffic.
For example, to arbitrate transmissions and avoid collisions, 802.11 specifies a distributed coordination function (DCF) based on carrier sense multiple access with collision avoidance (CSMA/CA). When a station receives a new packet for transmission from the upper layer, it selects a backoff counter (BC) uniformly at random from {0, . . . , CW−1}, where CW denotes the contention window and is initially set to a minimum value CWmin. The backoff mechanism employs a discrete time scale; for each time slot during which the medium is sensed to be idle (i.e. below the carrier-sensing threshold), the station decreases its backoff counter BC by 1. It is to be noted that in the present description, the term mechanism is being used in its metaphorical sense, meaning a means or technique. When the medium is sensed busy, the station freezes its backoff counter until the medium is sensed idle again for a duration equal to DCF interframe space (DIFS). The station transmits when the backoff counter reaches 0. If the destination station successfully receives the frame, it waits for a duration equal to short interframe space (SIFS) and replies with an acknowledgement (ACK). If there is a collision (detected by a missing ACK), this is interpreted as contention, and the transmitting station reduces its aggressiveness by doubling CW (up to a CWmax value). It then repeats the process.
The time slot duration must last long enough to perform reliable carrier-sensing (i.e. measure the energy level), switch the radio frequency (RF) front-end from receiving to transmitting, and account for possible propagation delays. It appears that these durations are mostly incompressible; for instance the 802.11a/g/n/ac amendments have been using time slot durations given by tslot=9 μs for more than a decade. Similarly, SIFS needs to account for the time required to process the incoming frame and to switch the mode of the RF front-end to transmit the ACK. 802.11a/n/ac use SIFS durations given by tSIFS=16 μs. These time constraints also propagate to DIFS, which is set to SIFS+2 time slots and is equal to tDIFS=34 μs for 802.11a/n/ac. Finally, each frame starts with the transmission of a PHY preamble, which is required to detect and to decode frame transmissions, as well as to set the spectrum and modulation parameters. In total, 802.11ac uses PHY preambles lasting for durations of tPHY=44 μs.
A normalized throughput or efficiency of a media access control (MAC) protocol can be defined as the product of (i) the fraction of time and (ii) the fraction of spectrum that are used for successful transmission of payload traffic. Since 802.11, for example, uses 100% of its channel, its efficiency is only determined by its time-domain operation. To analyze the efficiency of 802.11 as a function of the PHY rate, a following simple analytical model can be used. When there is only one transmitting station (and thus no collision), the average value of BC, which is denoted by BC, is given by BC=(CWmin−1)/2. The efficiency can thus be easily computed aseff802.11=tdata/(tDIFS+BCtslot+tPHY+tdata+tSIFS+tACK),where tdata denotes the time required to transmit the payload and tACK is the total time required to send the ACK. Although faster transmission rates reduce the total time required for transmitting a frame, they exacerbate the time-domain overheads explained above, because proportionally, the time domain overheads now take longer than with slower transmission rates. The efficiency is often below 10% with 802.11ac, for example.
It is an object of the present invention to overcome the problems related to the efficient use of network resources in wireless communication systems.